An engine with the ability to vary its compression ratio during operation can significantly improve efficiency and power density for compression-ignited (CI), stoichiometric spark-ignited (SSI), homogeneous charge compression-ignited (HCCI) and lean combustion spark-ignited (LCSI) applications. Variable compression ratio (VCR) abilities allow an engine's compression ratio to be lowered during high load conditions to prevent detonation or to limit peak cylinder pressure and allow the compression ratio to be raised to improve thermal efficiency at lower load conditions. Variable compression ratio can also be used to phase or supplement the phasing of combustion in homogeneous charge compression engines and to broaden the range of air-fuel ratio that can be used in lean combustion spark-ignited engines. In nearly all engine applications, variable compression ratio is especially beneficial when used in combination with supercharging, where the combination of variable compression ratio and supercharging substantially multiplies the benefits of both features.
In spite of their benefit and applications, engines with variable compression ratio abilities have not been used in commercial applications due to issues of extreme complexity, lack of long-term durability and prohibitively high costs. Several approaches to varying compression ratio in conventional slider-crank engines have been proposed and in some cases have been implemented. One type of variable compression ratio device for use in slider-crank engines is shown in the following publications: PCT Publication No. WO 92/09798, PCT Publication No. WO 92/09799, U.S. Pat. No. 5,329,893; and U.S. Pat. No. 5,443,043, all of which are assigned to Saab. The Saab design includes a traditional in-line slider-crank engine in which the head and cylinder bank are tilted to vary compression ratio. The design preferably also includes an external supercharger for providing boost air to the engine. Using stoichiometric spark-ignition, the Saab variable compression ratio engine has demonstrated a 30% improvement in fuel economy in combined city and highway driving cycles. In spite of its benefits, Saab's pivoting head design is cumbersome and complex, has potential sealing issues and is prohibitively expensive. Other attempts to vary compression ratio in conventional slider-crank engines are generally inferior to the Saab technique for various reasons.
Another means of varying compression ratio in slider-crank engines is achieved through variable valve timing. Variable valve timing is a very good technology for extending an engine's torque curve over a broad range of engine speed and for some Miller cycle variable compression ratio applications. However, the usefulness of variable valve timing for other variable compression ratio applications is very limited. Varying compression ratio with variable valve timing relies on decreasing the effective stroke of the engine to lower compression ratio. This results in significant penalties in the effective displacement of the engine as compression ratio is lowered. In nearly all variable compression ratio applications, compression ratio must be lowered when peak power is needed. A reduction in the effective engine displacement at this time significantly reduces the peak power capability of the engine and usually offsets any benefits that can be gained by a varying compression ratio.
In contrast to conventional slider-crank engines, single-ended barrel engines by nature provide a structure that is better suited to utilize a simple and effective means of varying compression ratio. The engine structure of a single-ended barrel engine allows the engine's compression ratio to be easily and simply varied by axially changing the position of the engine's central track or cam drive mechanism. By moving the track axially, the pistons can be brought closer to or further away from top dead center (TDC), thus, varying the engine's compression ratio. This method of varying compression ratio is both durable and inexpensive and is more effective than variable valve timing methods of varying compression ratio in most applications.
A single-ended barrel engine design is also advantageous because it allows piston motion to be independently optimized for the intake, compression, combustion and exhaust cycles. This level of optimized piston motion is not possible in slider-crank engines, which are restricted to sinusoidal piston motion or in double-ended barrel engines, which cannot independently optimize piston motion for each cycle.
While single-ended barrel engines provide a simple and inexpensive means of varying compression ratio and the ability independently optimize piston motion for various engine cycles, prior art single-ended barrel engines with these abilities have yet to demonstrate a piston structure that is both structurally and kinematically feasible at normal engine speeds. Prior art single-ended barrel engines employ single-ended or double-ended, single roller piston designs that lack critical crosshead and roller support qualities needed for a feasible design.